Plywood for CNC Work

[Brad Shipton]

I have only had my CNC for about 4mo or so now. I have never been a big fan of MDF or PB, but the more I work with my new machine the more I understand the benefits. The plywood I buy at the hardware store (baltic birch, domestic's or imports) is simply not usable for much of what I want to use the CNC for. I completed one project where I had a minor mistake in my tool paths, so I had to buy an extra sheet to recut some parts. I wound up milling the new plywood sheet to match the thickness of the parts already made so they would fit into the dado slots. I am not prepared to give up plywood (getting closer), but it seems I will have to start ordering better material. I live in a small community, so I have to order from a distance. To keep the costs reasonable I have to buy from a wholesaler. I found a plywood Columbia Forests makes that I would like to try, but it is not a stock item so I cannot measure any sample sheets. Since it is a custom order I will need to purchase a complete lift of 30 sheets. Its a bit on the expensive side, so I am wondering if others have tried the better plywood grades and if so, did it save enough of the headaches to justify the cost.

I am looking at the Purebond J Classic Core: http://www.columbiaforestproducts.com/product/classic-core/

I did notice the MDF cores. It makes sense that it will be easier for them to produce a more consistent thickness with those, so I will have to live with that. Maybe it is time to move past my silly hang ups about PB? Some of the thermo fused laminate options are pretty awesome, and the idea of deleting the finish step is pretty darn enticing.

Thanks

[Art Mann]

When you say "PB", do you mean particle board? I would not use that material for any CNC router project. I would only use plywood if I were cutting out cabinet panels. I mostly use PVC sheets for signs and hardwoods for gift store type products. What is it you want to use plywood for? If it were me and I needed to cut a project using plywood, I would only use Baltic Birch plywood from a high quality dealer. My Lowes and Home Depot stores don't carry genuine imported Baltic Birch. You may be talking about just plain Birch domestic plywood which is mostly very bad quality purchased from any big box store. The internal plyies are typically full of voids and the sheets are subject to warping just sitting in storage. I am not familiar with domestic Baltic Birch plywood. I thought it was called "Baltic" because it was imported from the Baltic countries of Europe.

[Gerry Grzadzinski]

That Classic Core is really nice stuff. Imo, far superior to standard plywood. Very consistent thickness.

I've been programming and running industrial cnc routers for almost 20 years, and have cut thousands of sheets of plywood, mdf, particle board, and other products with a variety of veneers and other surfaces.
PB and MDF panels make good substrates, and will last indefinitely if you use them properly, and don't poor water on them.

The only "standard" plywood that will have consistent thickness is baltic birch, or any similar product with a different name (all hardwood layers), like Appleply.

I've cut jobs that used 200-300 sheets of plywood and the thickness would change every 25-30 sheets or so. If I'm cutting dadoes for plywood, I make them .015"-.020" wider and live with a slightly loose fit.


PS. My wife has family in Dawson Creek, and I've been there several times. :) I was up near there last week.

[Brad Shipton]

I am mainly making cabinetry and other indoor furniture pieces. In many cases I am using either dado joints or lamello zeta connectors. I did mean particle board with the PB acronym. The core for much of the melamine on the market is particle board. In the PB specs they indicate a thickness tolerance of .008" and the plywood I have bought here has varied by as much as .04". The classic core from Columbia Forests indicates a 3/64" thickness variance with their standard classic core, so that is in line with what I am measuring now that I check more frequently. MDF is my least favorite material for quite a few reasons, but I have wound up using it many times for nice veneers. It is a pretty good substrate for veneers.

I tried baltic birch on a cabinet for my shaper cutters, and it was not great. Maybe I will give it another go. I suspect the local store focuses a lot more on price than quality when they buy at the head office. I have bought quantities from a wholesaler before, but that was before I had the CNC, and I didn't pay that close of attention to the thickness since I fiddled with my dados as I was cutting the parts instead of in a computer model. I did find the 60" sheets to be a nuisance since I am very limited on space now where I cut the parts and my table is only 4x8.

When I mentioned domestic, I didn't intend to mean the baltic birch was from North America. I too am sure it comes from the baltics. The domestics I have tried so far are just good ol birch and maple sheets from Canada.

In all honesty I have not worked with a lot of sheet goods since resuming with woodworking about 10years ago. I have been making mainly architectural items like floors, inlays, doors, stairs, and deck parts using hardwoods. I have been planning my new kitchen lately, and if it made sense I would make the boxes using hardwood. I know that is a bad idea for stability reasons, build time, and costs, but it crosses my mind sometimes since I know of bespoke cabinet makers in the UK doing that. The idea of melamine in my kitchen is not something I like, but it is hard not to ponder that when you slap down a sheet on the table and it lays perfectly flat while being as close to a consistent thickness as one can expect with a wood material. It also seems strong enough for 90% of residential applications so long as you do not drown it in water.

Gerry, that is very helpful. I am a bit of a fuss budget so the idea of over sizing the dados for material short comings might take me some time to get used to. I have looked at the joints in a few of my little CNC test projects lately and thought I could do the same with my shaper and table saw. Granted, they were faster on the CNC. I suppose I will need to order some samples from Columbia Forests to take a look at myself before dropping $150/sheet after I include the shipping. I have a lot of time to decide yet. I have two projects ahead of this, and then I have to turn a wood slab I have coming from Washington into a conference table for our office. Nothing with adding the CNC was overly easy, but the material aspect is getting more difficult now since I do not have a local supplier that has much CNC experience. Even the wholesaler was a bit reluctant when I started asking about plywood for CNC use. I suspect the vast majority of their commercial customers do not use a lot of plywood.

Gerry, I have lived in Dawson all my life and much of my family still lives here. It is a fine little town and we get visited by lots of Americans in the summer on their trip to Alaska. Sometimes it would be nicer to be where I could look at larger stores, but the trucking companies are surprisingly okay with shipping things to us. :D

[Gary Campbell]

Brad...
In most cases you will not be happy with the quality of consumer grade sheetgoods sold at the big box stores. Same goes for most imported products. To me, Baltic Birch comes from Russia and has cross plies of hardwood. For ease of use and quality Columbia Purebond is good, and is a good idea. The tolerance you state seems over my own experience, but is what you should expect from todays sheetgoods.

IF you are going to process cabinet panels from sheetgoods with a cnc you will need to adopt a method that is contusive for sheetgoods and cnc machining. The full dado is a great joint, but nowhere near the best for plywood which has thickness variations greater than that type of joint can tolerate, especially when too loose. Look into the tenon dado joint. Made for CNC. You machine an undersized dado somewhere around 50% of the material thickness in width and depth and then machine a tenon to fit on the end of the panel. Works like a charm and is used in most high end cabinet software brands, albeit name differently.

[dan nelson]

Not much input on your plywood choice but have you considered rethinking your dados?
Ive found that milling a 3/8x 3/8 or 9.525 dado into the sides and machining a matching tongue thickness for the tops and bottoms to slide into the dados up for the tops and bottoms to be blind dados. Meaning cut the side dados short of the face and notch the tops and bottoms to match. this probably works better on face frame type cabinets than frameless for any variation thicknesses in your material.

[Brad Shipton]

I really appreciate your comments Gary. It has been difficult to decide since the local shop looks at me with a blank stare when I start commenting about thickness variations and sheet bow. It is interesting to note that they stock the imports in lift quantities, but they only stock domestics in small quantities. The nearest Home Depot does not stock Purebond anymore. Years ago they did. I guess I will have to plan on ordering in larger quantities from a wholesaler. I guess my cabinet backs will be the same as the sides since I do not have space for multiple lifts.

I see the http://www.tenoncam.com/ joint. That might be a good option and it keeps the tool path simple.

Dan, I tried blind dados, but with the material thickness variations I noticed small overhangs in a few cases. Yes, the overhang was very small, but I didn't buy a CNC to see that. With better plywood that option will be fine. I suppose part of the problem is getting over the cost of the better sheets. Eliminating finishing with the melamine would be great too. Finishing has always been a chore for me.

[Gary Campbell]

Brad...
A few facts you may want to consider:
Prefinished plywood was initially made for the CNC industry. Router table and P to P machines.
The joint you show requires a special bit and system. Dan and I are talking about those that can be cut with inexpensive standard bits.
Until you get into CNC you may not notice sheet variations, other than the difference in "nominal" thickness
"Blind" as it refers to dados means not machining the dado to the panel edge. For those that use edgebanders.

[Brad Shipton]

I noticed that system requires a license and the bits are expensive. I have had custom shaper cutters made in the past, and I wouldn't put it past myself to order custom cutters for this machine at some point. I can automate the tool paths in solidworks once I define the tool geometry. Figuring out the math and creating the models is simple.

Thanks for correcting my terminology with respect to the dado. I was talking about a tenon joint that went thru to the edge of the panel. I will try building a few more test boxes with the tenon joint you and Dan mentioned before ordering the expensive material. Each time I try a new project I keep learning different things about the differences between conventional and the CNC. I know it will take much more practice.

[Gerry Grzadzinski]

I've never seen that TenonCAM before, but I certainly wouldn't recommend it.
Looking through their site, it costs between $5000-$15000, depending on the software capabilities. I'd imagine that it must use a 1" diameter bit, or close to it. And the bits can not be resharpened.

The only real advantage that I can see is that it doesn't require a tool change. With the method that Gary described above, you probably want to cut the tenons and mortise/dados with a downcut bit, and cut the parts themselves with a compression spiral.

[Brad Shipton]

Ok, it was not my best the best link to use in the morning. They use a 1/2" compression bit with a recess to create the tenon. The cutters are $225 each and they suggest they last for 1 - 200 sheets of plywood depending on the material. One cannot buy the bits without a license, so I would have to get one made unless they have a really good patent. I could create the toolpath in our software, so I would not need that. I have a tool changer and speed is not all that critical to my wee shop.

[Gerry Grzadzinski]

Looking again, I see that they still use multiple bits.
And the 1/2" bit only works with 1/2" material. Thicker material requires a 5/8" bit.

To me, it looks like a solution to a problem that doesn't exist.

[Brad Shipton]

Yes, I agree with that thinking about it more now. I will keep it simple like you all suggest and see what I can find for a more reliable supplier. Fun times.

[Gary Campbell]

Brad...
+1 to what Gerry states in both his posts.

Here is a VCPro screen snip of the type of dados I was referring to. Like Gerry says, they are cut best with a downspiral and compression, but they can be cut with a single 3/8" compression if needed. On the left/bottom a top/bottom/fixed shelf (horizontal) is shown and on the right top would be a left/right side or vertical

[Brad Shipton]

Yes, that is what I thought you meant. It solves the joint fit problem, but I still think I need to find a better plywood source. I think I will order the Hardwood Plywood and Veneer Association (HPVA) spec book to get a better understanding of the requirements for hardwood sheet products. All I have found online is brief excerpts.

[Gerry Grzadzinski]

Just because it meets their specs won't guarantee that it's any good. According to Columbia's website, the HPVA thickness tolerance is +0, -3/64".
We do a recurring job that uses about 40 sheets of red oak plywood. I buy from 3 different manufacturers. None are really all that great. I find that plywood with a poplar core tends to be a little better than some of the others. But the real problem that I see a lot is de-laminating of the face veneers. Splintery woods like oak are the worst offenders.

Personally, if I were buying plywood for personal projects, I'd by the Purebond Classic Core. I've used it and it's nice. A major benefit is that it tends to be a lot flatter than regular plywood, which is nice for vacuum holding.

It looks like the J classic core is a more consistent thickness, based on their website.

[Keith Winter]

Columbia Forests is the supplier for some of the home depot plywood. If you have one within a couple hours you could call in advance to make sure it was there and load up your pickup truck vs special ordering. Here are a couple examples:

http://www.homedepot.com/p/Columbia-Forest-Products-3-4-in-x-4-ft-x-8-ft-PureBond-Birch-Plywood-FSC-Certified-332941/202887466
http://www.homedepot.com/p/Columbia-Forest-Products-23-32-in-x-4-ft-x-8-ft-PureBond-Poplar-Plywood-770579/203335841

Google "home depot purebond plywood" for even more sizes and wood type options.


I have only had my CNC for about 4mo or so now. I have never been a big fan of MDF or PB, but the more I work with my new machine the more I understand the benefits. The plywood I buy at the hardware store (baltic birch, domestic's or imports) is simply not usable for much of what I want to use the CNC for. I completed one project where I had a minor mistake in my tool paths, so I had to buy an extra sheet to recut some parts. I wound up milling the new plywood sheet to match the thickness of the parts already made so they would fit into the dado slots. I am not prepared to give up plywood (getting closer), but it seems I will have to start ordering better material. I live in a small community, so I have to order from a distance. To keep the costs reasonable I have to buy from a wholesaler. I found a plywood Columbia Forests makes that I would like to try, but it is not a stock item so I cannot measure any sample sheets. Since it is a custom order I will need to purchase a complete lift of 30 sheets. Its a bit on the expensive side, so I am wondering if others have tried the better plywood grades and if so, did it save enough of the headaches to justify the cost.

I am looking at the Purebond J Classic Core: http://www.columbiaforestproducts.com/product/classic-core/

I did notice the MDF cores. It makes sense that it will be easier for them to produce a more consistent thickness with those, so I will have to live with that. Maybe it is time to move past my silly hang ups about PB? Some of the thermo fused laminate options are pretty awesome, and the idea of deleting the finish step is pretty darn enticing.

Thanks

[Brad Shipton]

I requested the nearest Home Depot in Canada to order a lift of the J Grade, but they could not or did not want to get the material. The product is too expensive retail anyway. I have a quote from a wholesaler, but the J Grade is still a special order where I have to purchase a full lift. I have to finish some projects to make space for a full lift. The nearest home depot has scaled back what they carry for sheet goods a lot over the years. I have checked the Home Depot in the nearest large cities and it does not seem to be much different as far as what they stock. Years ago they stocked a few different Columbia products, but now they do not and when you ask them to order material they prefer to sell in complete lifts.

[Gary Campbell]

Brad...
Are there any medium to large cabinet shops in your area? You may be able to add smaller quantities to orders they place thru their suppliers. If they are not CNC equipped, you may get some cutting out of the deal.

[Brad Shipton]

There are a few I could talk to in cities nearby. It is a lot better in person since you can get past the front desk person and get to talk to someone that uses the equipment. The only other CNC in my town is at the college, and when I looked at their machine I didn't get the impression it was being used very much. Admittedly, I might be a bit too fussy. I know I can work around the problem since most of my projects are small, but I find it a bit annoying. I have to order some melamine for an office project, and I will order the stock Columbia Forests combi-core to try. I picked up some maple plywood from the company in Town yesterday. The veneer core material will be taken back, but the other two sheets with mdf core layers seems pretty respectable.

I have been toying with the idea of custom cutting. It looks likely I will be making some anchor bolt templates for a few industrial compressor skids in a couple of weeks.

[Chris Vickers]

Look to see if you can find Garnica. I've been using it now for the past 6 months and love it. It is from Spain, true .75 thick plus or minus a bit just like all ply, but the reason I like it is, it has a thicker veneer face/back and 2 extra plys in the core. Very flat and it is 12 pounds lighter than the domestic. It is truly void free. Poplar core and routes excellent. We use it for all our cabinet shops we cut for unless they specify Columbia purebond.

[Brad Shipton]

I will look into the Garnica. It seems my closest contact is in Quebec.

I got some of the Purebond a few days ago with another order, and it looks promising in the grade they stock. The thickness difference along the edge of the sheets was within 10 thou, and the difference between the three sheets was around 15thou or so. That is a lot better than the worst case they suggested on their website.

Thanks again to all of you.